Thin film photovoltaic devices can include semiconductor material deposited over a substrate, for example, with a first semiconductor layer serving as a window layer and a second semiconductor layer serving as an absorber layer. The semiconductor window layer, for example, a cadmium sulfide layer, can allow the penetration of solar radiation to the absorber layer, for example, a cadmium telluride layer, for conversion of solar energy to electricity.
During conversion of solar energy to electricity in the photovoltaic device, some minority electron carriers penetrate through the absorber layer to a back contact adjacent to the semiconductor layer where they combine with hole carriers, causing power dissipation inside the device, thereby reducing power conversion efficiency. To eliminate power dissipation, an additional semiconductor layer may be deposited between the semiconductor absorber layer and the back contact layer as a barrier or reflector against minority electron carrier diffusion. The reflector layer is made of a semiconductor material with electron affinity lower than that of the absorber layer, which forces electron carrier flow back toward the electron absorber layer, minimizing recombination at the back contact.
Although the reflector layer should reduce power dissipation and increase power conversion efficiency, lattice mismatch between the reflector layer and the absorber layer can partially negate this benefit. Semiconductor materials contain a lattice, or a periodic arrangement of atoms specific to a given material. Lattice mismatching refers to a situation wherein two materials featuring different lattice constants (a parameter defining the unit cell of a crystal lattice, that is, the length of an edge of the cell or an angle between edges) are brought together by deposition of one material on top of another. In general, lattice mismatch can cause misorientation of film growth, film cracking, and creation of point defects. In typical photovoltaic devices, lattice mismatching can occur, for example, between the semiconductor absorber layer and the semiconductor reflector layer. Lattice mismatch between the semiconductor absorber layer and the semiconductor reflector reduces desired electron reflection. Power dissipation within the photovoltaic device continues, thereby negating the desired benefits of the reflector layer and reducing power conversion efficiency.
An improved photovoltaic device and method for manufacturing the same that mitigates against lattice mismatching between the semiconductor absorber layer and the semiconductor reflector layer is desirable.